Isomorphously substituted molecular sieve membranes

ABSTRACT

Zeolite membranes that can be used to continuously separate components of mixtures are disclosed. The zeolite membranes are prepared by isomorphous substitution, which allows systematic modification of the zeolite surface and pore structure. Through proper selection of the basic zeolite framework structure and compensating cations, isomorphous substitution permits high separation selectivity without many of the problems associated with zeolite post-synthesis treatments. The inventive method for preparing zeolite membranes is alkali-free and is much simpler than prior methods for making acid hydrogen zeolite membranes, which can be used as catalysts in membrane reactors.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.60/177,542, filed Jan. 21, 2000, which is herein incorporated byreference in its entirety for all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to membranes having molecular sieveproperties and/or catalytic activity and to methods for producing andusing the membranes, and more particularly, to isomorphously substitutedzeolite membranes and their use in selective separations of moleculesand in catalytic membrane reactors.

2. Discussion

Zeolites are crystalline aluminosilicates of Group 1 and Group 2elements. Their basic structural framework can be viewed as athree-dimensional network of SiO₄ and [AlO₄]⁻ tetrahedra, which arelinked by oxygen atoms. The structural framework encloses cavities anddefines channels or pores that are substantially uniform in size withina specific zeolite. As discussed below, large ions (compensatingcations, M) and water molecules occupy some of the cavities and haveconsiderable freedom of movement within the zeolite lattice, whichallows zeolites to perform ion exchange processes and reversibledehydration.

Because the zeolite pores are sized to accept molecules of certaindimensions for adsorption while rejecting molecules of largerdimensions, these molecules have come to be known as “molecular sieves.”Zeolites have been used commercially in ways that that take advantage ofthese properties, including adsorption separation processes andshape-selective catalytic processes.

Most commercial applications use zeolites in the form of granules orpellets. Zeolite granules exhibit high porosity and have a uniform poresize between about 0.3 and 1.2 nm that is dependent on the specificzeolite structure. Such granules are the catalysts of choice for thepetrochemical industry. Shape-selective effects are possible because thecatalytic sites are accessible only within the pores of a zeolitestructure, and only those reactant molecules, transition states,intermediates, and/or product molecules with dimensions below a certaincritical size can be adsorbed into this pore system. Shape-selectivecatalysis combines the molecular sieving effect with a catalyzedreaction.

Recently, zeolite membranes have been used to conduct molecularseparations. Generally, a membrane can be defined as a semi-permeablebarrier between two phases that is capable of restricting the movementof molecules across it in a very specific manner. The semi-permeablenature of the barrier is essential to obtaining an effective separation.A wide variety of molecular materials, mostly organic polymers, havebeen found to be suitable for use as membranes. However, organic polymermembranes have relatively short service lives because of theirsensitivity to solvents and low stability at high temperatures.

Because of their superior thermal, chemical, and mechanical properties,zeolite membranes have substantial advantages over organic polymermembranes. The pore size is uniform within a specific zeolite material,and the pore size of a zeolite membrane can be synthetically tuned bychoosing an appropriate zeolite structure and/or by exchangingcompensating cations of different diameters. The hydrophilic/hydrophobicnature of a zeolite can be modified by changing the substituted metal(Me) in the framework and the Si/Me ratio. The basic/acidic nature ofthe zeolite can be modified by exchanging alkaline cations with protons.Moreover, zeolite membranes can be used for catalytic membrane reactorsbecause they combine heterogeneous catalytic sites with membranes thatallow only one component of a mixture to selectively permeate across themembrane. Zeolites can be considered as originating from a SiO₂ latticein which Al³⁺ is isomorphously substituted for a portion oftetrahedrally coordinated Si⁴⁺, and can be represented by the formula:

 M _(x/n)·[(AlO₂)_(x)·(SiO₂)_(y) ]·zH ₂ O  I

where M represent a compensating cation with valence n, y is a numbergreater than or equal to x, and z is a number between about 10 and10,000. In an isomorphous substitution, a second (different) elementreplaces some (or all) of an original element of the crystallinelattice. The second element has similar cation radius and coordinationrequirements as the original element so that the same basic crystallinestructure is maintained.

Because aluminum is trivalent, every tetrahedral [AlO₄] unit carries anegative charge. Consequently, the substitution of aluminum for silicongenerates an excess negative charge in the zeolite lattice that must becompensated by cations. These compensating cations may be exchangeable.Accordingly, the ion-exchange capacity of a zeolite is enhanced as thealuminum content is increased. Acid hydrogen forms of zeolites haveprotons that are loosely attached to their framework structure in lieuof inorganic compensating cations, and these proton sites function asBrönsted acids. Thus, the number of protons that may be attached to thezeolite framework is greater in zeolites having greater aluminumcontent. Consequently, increases in the aluminum content of a zeolitecan result in additional Brönsted acid sites. Zeolites having additionalcatalytic sites exhibit greater activity in acid catalyzed reactions.Thus, the ion exchange and the catalytic properties of a specificzeolite depend on its chemical composition and, more particularly, onits Si/Al ratio.

Zeolites represented by formula I are often described in terms of theirSi/Al ratio, because certain properties of zeolites appear to vary withSi/Al ratio. In an extreme case in which substantially all of thelattice ions are silicon, zeolites can have Si/Al ratios that approachinfinity (e.g., silicalite-1). Such zeolites do not have a net negativeframework charge and therefore do not contain compensating cations. As aconsequence, these zeolites have no ion exchange capacity, cannot beacidic, and exhibit a high degree of hydrophobicity. These highlysiliceous zeolites are organophilic and have been used for the selectiveadsorption of volatile organic compounds. Zeolites with Si/Al ratios aslow as 0.5 have also been made (e.g. bicchulite).

With zeolite membranes, separation is thought to occur through at leastthree different, nonexclusive mechanisms, which are based on differencesin component diffusion, on molecular sieving or size exclusion, and onpreferential adsorption. Thus, two or more different types of moleculesmay access the pore system of the zeolite membrane, but their diffusionrates through the pores may vary because each type of molecule interactsdifferently with the zeolite surface and pore structure. Additionally,molecular sieving may occur when one type of molecule can access thezeolite membrane pore system, but a different type of molecule cannotbecause of its larger size. Finally, the pore system of the zeolitemembrane may preferentially adsorb a first molecule, which blocks entryof a second, different molecule into the pore system. Because moleculeswith different sizes and shapes have different diffusivities, highseparation selectivities have been reported for n-C₄H₁₀/i-C₄H₁₀, andn-C₆H₁₄/3-methyl pentane mixtures. Likewise, high separationselectivities based on molecular sieving were obtained for CH₄/i-C₈,n-C₆/2,2 dimethylbutane, and p-/o-xylene mixtures. Selectivities havealso been attributed to differences in adsorption properties.

It is important to recognize that adsorptive separation processes ongranular molecular sieves are two-step batch processes involvingsuccessive adsorption and desorption of molecules. In contrast, membraneseparations are continuous processes that are accomplished by applying adriving force across the membrane (e.g., pressure gradient,concentration gradient, or temperature gradient). Thus, membraneseparations do not require regeneration of the active sites in themembrane by desorption. Instead, a vapor-phase feed stream iscontinuously applied to one side of the membrane while purified productis continuously removed from another (permeate) side of the membrane.Because zeolite membranes allow continuous separation of multi-componentmixtures, they offer significant advantages over zeolite granules,including less capital expenditure for equipment and fewer processingsteps.

Despite the perceived advantages of zeolite membranes, their use inseparations and catalysis poses significant challenges. Because theirability to separate molecules depends on surface properties and porestructure, which can vary significantly among different types ofzeolites, many zeolite membranes demonstrate limited selectivity forseparating mixtures of molecular components. Previous attempts toimprove membrane performance have met with limited success. For example,post-synthesis treatments such as CVD modification or coke depositionmay block access to the zeolite pore system and/or reduce pore entrancediameters, thereby decreasing flux through the membrane.

Although the acid hydrogen form of zeolite membranes is useful forcatalytic membrane reactors, synthesis of acidic zeolite membranes is acomplex process. Conventional synthesis of acid zeolite membranerequires the use of alkali metal hydroxides. Subsequent steps involveacid treatment or ion exchange with an ammonium salt solution, followedby thermal decomposition of the ammonium ion to obtain the acid hydrogenform of zeolite membranes.

The present invention overcomes, or at least mitigates, one or more ofthe problems set forth above.

SUMMARY OF THE INVENTION

The present invention provides zeolite membranes that can be used tocontinuously separate components of mixtures. The zeolite membranes areprepared by isomorphous substitution, which allows systematicmodification of the zeolite surface and pore structure. Through properselection of the basic zeolite framework structure and compensatingcations, isomorphous substitution permits high separation selectivitywithout many of the problems associated with zeolite post-synthesistreatments. The inventive method for preparing zeolite membranes isalkali-free and is much simpler than prior methods for making acidhydrogen zeolite membranes, which can be used as catalysts in membranereactors.

To achieve the foregoing and other objects, one aspect of the presentinvention provides a membrane comprising a layer of an isomorphouslysubstituted zeolite. The isomorphously substituted zeolite membrane canbe represented by the formula:

x ₁ M ₁ ^(n1+) ·x ₂ M ₂ ^(n2+)·[(y ₁ T ₁ ·y ₂ T ₂ ·y ₃ T ₃ . . .)O_(2(y)₁ _(+y) ₂ _(+y) ₃ _(+ . . .)) ]·z ₁ A ₁ ·z ₂ A ₂ . . . ;  II

wherein T₁ is tetrahedrally coordinated Si, T₂ is a tetrahedrallycoordinated element and is B, Ge, Ga or Fe or combinations thereof Inaddition, T₃ is tetrahedrally coordinated Al, M₁ and M₂ are compensatingcations having valences n1 and n2, respectively, A₁ and A₂ are adsorbedspecies located within the zeolite, and x₁, X₂, y₁, Y₂, Y₃, Z₁, and Z₂are stoichiometric coefficients. The present invention also contemplatesan acid hydrogen form of the isomorphously substituted membranes havingprotons attached to the zeolite framework in lieu of inorganiccompensating cations.

The membranes of the present invention can be used in numerousprocesses, including component separations based on at least onemolecular property selected from size, shape, and polarity. Inparticular, the claimed membranes are capable of separatingnon-condensable gaseous mixtures, condensable organic vapors, water froma mineral acid solution, and one or more components of aqueous organicmixtures. In addition, some of the claimed membranes can be used tocatalyze chemical reactions.

The surface properties and the pore structure of the zeolite membranescan be altered by appropriate selection of membrane components, allowingsuperior separations for a wide variety of mixtures. In one embodiment,the zeolite membrane is substantially free of alkali metal hydroxides.In another embodiment, y₃ is substantially equal to zero, and thezeolite membrane is substantially free of aluminum. The ratio of T₁/T₂is generally between about 12 and about 600, and more typically, betweenabout 12.5 and about 100.

Another aspect of the present invention provides an article ofmanufacture comprising a porous support and a membrane layer disposed onthe porous support. The membrane layer comprises an isomorphouslysubstituted zeolite having a composition that can be represented byformula II described above. The membrane may be substantially free ofaluminum, with y₃ of formula II substantially equal to zero, and may beformed in-situ on and within the pores of the support. In oneembodiment, the porous support has the form of a container, and themembrane is disposed on the interior surface of the container. Usefulporous supports include tubes made of stainless steel, α-alumina, orβ-alumina.

A further aspect of the present invention provides an apparatus forseparating one or more components from a mixture. The apparatus includesat least one membrane unit, a device for introducing the multi-componentmixture into the membrane unit, and a device for removing the componentsfrom the membrane unit. The membrane unit includes a porous support anda membrane layer disposed on the porous support. The membrane layercomprises an isomorphously substituted zeolite having a composition thatcan be represented by formula II. The apparatus may include a pluralityof membrane units to enable rapid processing of large volumes of amulti-component feed.

Still another aspect of the present invention provides a method ofmaking an isomorphously substituted zeolite membrane. The methodincludes preparing a porous support and contacting the porous supportwith an aqueous zeolite-forming gel. The gel is substantially free ofalkali hydroxides and includes silica, a quaternary organic ammoniumtemplate, and a source of ions. Useful ions include Al⁺³, Ge⁺⁴, Fe⁺³,Ga⁺³ or B⁺³ or combinations thereof. The method also includes heatingthe support and the gel to form (crystallize) a zeolite layer on theporous support, and calcining the zeolite layer to remove the template.The composition of the resulting zeolite layer can be represented byformula II.

In one embodiment of the method, the porous support is a containerhaving at least one opening and an inner surface, and the gel is placedinside the container. During the heating step, additional gel may beplaced in the container, and the container is sealed prior to heating.The heating step may be repeated one or more times to obtain a zeolitelayer on the support that is substantially impermeable to nitrogen.Acidic ZSM-5 membranes can be obtained directly without additional stepsinvolving ion exchange or acid treatment when the synthesis gel issubstantially free of alkali metal hydroxides.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a useful system for separatingcomponents of a mixture.

FIG. 2 shows a cross-sectional view of an embodiment of a membranemodule.

FIG. 3 shows a partial plan view of a selectively permeable portion of amembrane assembly.

FIG. 4 shows a block diagram of a method of making a zeolite membranelayer through in-situ synthesis on an inner surface of a porous support.

FIG. 5 shows a schematic view of an apparatus that can be used tocharacterize zeolite membranes by measuring single-gas and/or multi-gaspermeation rates at various temperatures.

FIG. 6 shows n-C₄H₁₀/i-C₄H₁₀ separation selectivity as a function oftemperature for three B-ZSM-5 membranes on stainless steel supports withdifferent Si/B molar ratios as indicated and for a silicalite-1 membraneon a stainless steel support.

FIG. 7 shows n-C₄H₁₀/i-C₄H₁₀ separation selectivity as a function oftemperature for three alkali free B-ZSM-5 membranes on α-aluminasupports with different Si/B molar ratios as indicated.

FIG. 8 shows n-C₄H₁₀/i-C₄H₁₀ mixture permeance as a function oftemperature for two alkali free B-ZSM-5 membranes on stainless steel andα-alumina supports; both membranes have Si/B molar ratios of 100.

FIG. 9 shows n-C₄H₁₀/i-C₄H₁₀ mixture permeance as a function oftemperature for two alkali free B-ZSM-5 membranes on stainless steel andα-alumina supports as indicated; each membrane has a Si/B molar ratio of12.5.

FIG. 10 shows n-C₄H₁₀/i-C₄H₁₀ mixture permeance as a function oftemperature for three alkali free B-ZSM-5 membranes prepared underidentical conditions on stainless steel supports; each membrane has aSi/B molar ratio of 100.

FIG. 11 shows n-C₄H₁₀/i-C₄H₁₀ mixture permeance and separationselectivity as functions of time for an alkali free B-ZSM-5 membraneprepared on an α-alumina support and having a Si/B molar ratio of 12.5;measurements were taken at 473 K.

FIG. 12 shows n-C₄H₁₀/i-C₄H₁₀ separation selectivity as a function oftemperature for alkali free B-ZSM-5, Al-ZSM-5, and silicalite-1membranes having various Si/Me molar ratios as indicated.

FIG. 13 shows n-C₄H₁₀/H₂ separation selectivity as a function oftemperature for silcalite-1 and substituted ZSM-5 zeolite membranesprepared on stainless steel supports.

FIG. 14 shows H₂/i-C₄H₁₀ separation selectivity as a functiontemperature for silcalite-1 and substituted ZSM-5 zeolite membranesprepared on stainless steel supports.

FIG. 15 shows n-hexane/2,2-DMB permeance and separation selectivity asfunctions of temperature for B-ZSM-5 zeolite membranes prepared onalumina and stainless steel supports.

FIG. 16 shows n-hexane/2,2-DMB permeance and separation selectivity asfunctions of temperature for silicalite-1 and B-ZSM-5 zeolite membranesprepared on stainless steel supports.

FIG. 17 shows p-xylene and o-xylene steady state fluxes as functions oftemperature for B-ZSM-5 zeolite membrane BZ1 and a feed partial pressureof 2.1 kPa per isomer.

FIG. 18 shows p-xylene/o-xylene steady state separation selectivity as afunction of temperature for B-ZSM-5 zeolite membrane BZ1 and a feedpartial pressure of 2.1 kPa per isomer.

FIG. 19 shows flux of p-xylene as a function of temperature for B-ZSM-5zeolite membrane BZ2 and various feed partial pressures.

FIG. 20 shows flux of o-xylene as a function of temperature for B-ZSM-5zeolite membrane BZ2 and various feed partial pressures.

FIG. 21 shows separation selectivity for p-xylene/o-xylene mixtures as afunction of temperature for B-ZSM-5 zeolite membrane BZ2 membrane andvarious feed partial pressures.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates a useful system 100 for separating oneor more components of a condensed-phase mixture 102 using isomorphouslysubstituted ZSM-5 zeolite membranes. Although the system 100 shown inFIG. 1 has been designed to separate components of liquid mixtures bypervaporation, it can be modified to separate mixtures comprised ofvapor-phase components by vapor permeation as well (compare FIG. 5).

The system 100 includes a membrane module 104 having an inlet 106, afirst outlet 108, and a second 110 outlet. A metering pump 112 drivesthe mixture 102 from a reservoir 114 to the membrane module 104 inlet106 through a first conduit 116. A section 118 of the conduit 116upstream of the membrane module 104 inlet 106 is optionally wrapped inheating tape to preheat the mixture 102. As described below, themembrane module 104 includes a zeolite membrane (not shown) thatseparates the feed stream 120 into a vapor-phase permeate stream 122—theportion of the feed stream 120 that passes through the zeolitemembrane—and a liquid-phase retentate stream 124. The permeate 122 andthe retentate 124 exit the module 104 through the first 108 and second110 outlets, respectively. The retentate 124 returns to the reservoir114 via the first conduit 116. Component concentrations in the feed 120and the permeate 122 streams can be measured by gas chromatography (GC),high-pressure liquid chromatography (HPLC), or by GC and HPLC.

As can be seen in FIG. 1, a vacuum pump 126 communicates with the firstoutlet 108 of the membrane module 104 via a second conduit 128 andprovides a pressure drop, which drives the permeate 122 through thezeolite membrane. An electronic gauge 130 monitors the pressure in thesecond conduit 128, which splits into a pair of conduits 132, 134downstream of the pressure gauge 130. Each of the conduits 132, 134thermally contacts separate cold traps 136, 138, which condense thepermeate 122 flowing within the second conduit 128. As depicted in FIG.1, condensed-phase permeate 140 collects in the bottoms 142, 144 ofU-shaped tubes 146, 148 that are immersed in liquid nitrogen baths 150,152. The U-shaped tubes 146, 148 comprise a portion of the permeate flowpath between the first outlet 108 of the membrane module 104 and thevacuum pump 126. The system 100 also includes numerous valves 154, 156,158, 160, 162, 164, which isolate the pressure gauge 130, the cold traps136, 138, the vacuum pump 126, and the reservoir 114.

Prior to separation, the vacuum pump 126 evacuates the permeate 122 sideof the membrane module 104. Once the permeate side 122 of the membranemodule 104 reaches a desired vacuum level, e.g., about 200 Pa absolutepressure, the valve 162 closes the fluid connection between the vacuumpump 126 and the first outlet 108 of the membrane module 104. Becausecondensed-phase permeate 122 occupies little volume, the vacuum level,as indicated by the electronic pressure gauge 130, ordinarily shouldchange little—a few hundred Pa, say—during a pervaporative separation.In some cases, however, non-condensable gases (e.g., nitrogen, oxygen,etc.) may enter the permeate 122 stream via the feed 120 stream orthrough leaks in the system 100. Over time, these gases may accumulate,reducing the vacuum level or increasing absolute pressure in thepermeate side 122 of the membrane module 104. In such cases, the vacuumpump 126 and valve 162 can be cycled to remove the non-condensablegases.

FIG. 2 shows a cross-sectional view of an embodiment of the membranemodule 104. The membrane module 104 includes a tubular membrane assembly170, having an elongated, selectively permeable portion 172, which isconnected at its ends 174, 176 to a pair of rigid, tubular end supports178, 180. The membrane assembly 170 is retained within a shell 182 madeof brass, stainless steel, or other rigid and chemically resistantmaterial. The shell 182 includes a body portion 184 and a pair ofremovable end caps 186, 188. The end supports 178, 180 of the membraneassembly 170 are substantially impermeable to fluids. The end supports178, 180 provide sealing surfaces 190, 192 for o-rings 194, 196 that arecaptured in grooves 198, 200 formed by opposing chamfered surfaces 202,204, 206, 208 of the shell's 182 body portion 184 and end caps 186, 188,respectively. The o-rings can be made of any inert material, includingsilicone-based polymers, and fluorinated elastomers such aspolytetrafluoroethylene (PTFE),vinylidenefluoride-hexafluoropropylene-tetrafluoroethylene terpolymer,and the like. Clamps, threaded fasteners, and the like (not shown)provide an axial compressive force sufficient to seal the o-rings 194,196 against the shell 182 and the end supports 178, 180 of the membraneassembly 170.

As indicated by an arrow 210 shown in FIG. 2, during pervaporation thefeed stream 120 enters the membrane module 104 through the inlet 108,and passes into a cavity 212 formed by an inner surface 214 of one ofthe end caps 186. From the cavity 212, the feed stream 120 enters aninterior portion 216 of the membrane assembly 170. There, one or morefeed 120 components flow radially through the selectively permeableportion 172 of the membrane assembly 170, as vapor, and collect in acavity 218 formed by an inner surface 220 of the shell 182 and an outersurface 222 of the membrane assembly 170. Components that cannot passthrough the selectively permeable portion 172 of the membrane assembly170 remain in the liquid phase, and flow axially into a cavity 224formed by an inner surface 226 of the second end cap 188. As shown byarrows 228, 230 the resulting permeate 122 and retentate 124 streamsexit the membrane module 104 through the first 108 and second 110outlets.

FIG. 3 shows a partial plan view of the selectively permeable portion172 of the membrane assembly 170. As can be seen in a cutaway 250 of themembrane assembly 170, the selectively permeable portion 172 includes azeolite membrane 252 layer or film disposed on an inner surface 254 of aporous support 256 layer. An outer surface 258 of the membrane layerdefines the interior portion 216 of the tubular membrane assembly 170.The porous support 256 should be able to carry the zeolite membrane 252layer and should be able to resist chemical attack by the components ofthe feed stream 120. As described below, the porous support 256 shouldalso be able to withstand reaction conditions during preparation of thezeolite membrane 252 layer. The porous support 252 typically has anaverage pore size similar to or larger than the pore size of the zeolitemembrane 252 layer. Useful porous supports 256 include stainless steel,α-Al₂O₃, γ-Al₂O₃, SiC, SiN₃, SiO₂, TiO₂, ZrO₂ and other inorganicoxides. Alumina supports are commercially available from a variety ofvendors. Stainless steel supports are mechanically robust and areparticularly useful for separating acidic mixtures because they canwithstand attack by concentrated acids.

Although the membrane assembly 170 shown in FIG. 2 and FIG. 3 isgenerally cylindrical, the membrane assembly 170 can assume anyconvenient shape. For example, the membrane assembly 170 can compriseone or more planar layers, or can have a cross-section normal to theretentate flow that is generally oval or polygonal. Furthermore, themembrane module 104 shown in FIG. 1 may include more than one tubularmembrane assembly 170, which can be connected in parallel to the feed120, permeate 122, and retentate 124 streams.

The zeolite membrane 252 layer provides a semi-permeable barrier betweenthe liquid-phase retentate 124 stream and the vapor-phase permeate 122stream within the membrane assembly 170. As a “semi-permeable barrier,”the zeolite membrane 252 layer is capable of selectively restricting themovement of molecules through the layer 252, which is essential toobtain an effective separation of the components of the feed stream 120by pervaporation. The zeolite membrane 252 layer's ability to separatecomponents of the feed stream 120 depends, at least in part, on theparticular zeolite's pore system, surface properties, and hence chemicalstructure.

Useful zeolites include silicalite-1, ZSM-5, and zeolite analogueshaving a SiO₂ crystalline lattice in which one or more elements otherthan aluminum have been isomorphously substituted for some of thetetrahedrally coordinated Si⁴⁺. These zeolite analogues can berepresented by the formula:

x ₁ M ₁ ^(n1+) ·x ₂ M ₂ ^(n2+)·[(y ₁ T ₁ ·y ₂ T ₂ ·y ₃ T ₃ . . .)O_(2(y)₁ _(+y) ₂ _(+y) ₃ _(+ . . .)) ]·z ₁ A ₁ ·z ₂ A ₂ . . . ;  II

where the expression in brackets corresponds to the frameworkcomposition and other terms represent species that reside in the poresof the framework structure. In formula II, M₁ and M₂ are compensatingcations with valences n1 and n2, respectively; T₁, T₂, and T₃ areelements occupying the tetrahedral positions of the framework; A₁ and A₂are adsorbed species located within the porous framework; and x₁, x₂,y₁, Y₂, z₁, and z₂ are stoichiometric coefficients. In general, thebracketed quantity will have a negative charge and T₃ is nonzero.

Because a metal or metalloid species (Me) has been isomorphouslysubstituted into tetrahedral positions of the zeolite framework, it ismore useful to describe the zeolites represented by formula II using aSi/Me ratio rather than a Si/Al ratio. Alternatively, this ratio can beexpressed in accordance with formula II, above, as a T₁/T₂ ratio, whereT₁ is Si and T₂ is B, Ge, Ga, Fe, or Al. Useful zeolites also includethose having more than one isomorphously substituted elementincorporated into the zeolite framework structure. Although zeolitestraditionally have been defined to include only those aluminosilicateshaving an ordered, three-dimensional microporous structure, as usedherein, the term “zeolite” also includes zeolite analogues having metalsother than aluminum that are isomorphously substituted at thetetrahedral sites.

Isomorphous substitution has been shown to affect the surface propertiesand the pore structure of zeolites. For example, silicalite-1 and ZSM-5have MFI structure, but silicalite-1 is composed of pure silica whileZSM-5 has aluminum substituted into a fraction of the silicon(tetrahedral) sites of the framework structure. It is known thatsilicalite-1 and ZSM-5 have different surface properties and porestructure due to changes in T—O—T bond angle and T—O bond length, whereT represents Si or Al. Therefore, isomorphous substitution within theframework structure of silicalite-1 or ZSM-5 should also produce changesin T—O—T bond angle and T—O bond length, where T now represents Si, Al,Ge, B, Fe, or Ga. These changes should affect the surface properties andpore structure of the zeolite and the separation performance of theresulting zeolite membrane. Furthermore, with isomorphous substitution,the zeolite surface changes from hydrophobic (silicalite-1) tohydrophilic (ZSM-5) and from non-acidic (silicalite-1) to stronglyacidic (ZSM-5). The Brönsted acid strength increases in the followingorder: silicalite-1, Ge-ZSM-5<B-ZSM-5<Fe-ZSM-5<Ga-ZSM-5<ZSM-5 (i.e.,Al—ZSM-5). In acid-catalyzed reactions, the catalytic activity ofisomorphously substituted zeolites should increase with Brönsted acidstrength. Since reaction selectivity also depends on zeolite acidstrength, the substituted zeolite membranes may be useful in catalyticmembrane reactors.

Isomorphous substitutes of silicon must accept a tetrahedralcoordination with oxygen. In addition to aluminum (cation radius of0.051 nm), suitable substitutes (i.e., T₂, T₃, etc. in formula II)include boron (0.023 nm), iron (0.064 nm), germanium (0.053 nm), andgallium (0.062 nm). Among these elements, Ge⁴⁺has a diameter closest tothose of Si⁴⁺(0.042 nm) and Al³⁺and thus substitutes more readily thanother tetravalent ions. The B³⁺ cation is much smaller than the othersubstituted cations, and it is less stable in the tetrahedral positionsaccording to the Pauling Rule, which holds that cations are stable inthe tetrahedral positions when the ratio of cation to oxygen radius is0.225-0.425. Because of this instability, boron may be partially removedfrom the zeolite framework during preparation of the zeolite membranelayer 252 (i.e., during calcination). This extra-framework boron, whichis located within the channels and on the external surface of zeolite,could affect membrane properties. Also, Fe³⁺ may be difficult toincorporate into the zeolite framework because of its large diameter.Consequently, some extra-framework Fe³⁺ may be present in the membraneas well. Other elements having similar cation radius and coordinationrequirements can also be isomorphously substituted into the zeolitestructural framework.

Since Fe and B are trivalent, they create acid sites in the zeoliteframework structure. In contrast, Ge is tetravalent and therefore doesnot create acid sites. Such differences in acidity may affect thepermeability of the membrane. When boron is substituted into thesilicalite-1 structure instead of aluminum, membranes can be preparedwith Si/B ratios as low as about 12. In contrast, Al-ZSM-5 membranes aredifficult to prepare with such low Si/Al ratios.

Referring once again to the drawings, FIG. 4 shows a block diagram of amethod 280 of making the zeolite membrane layer 252 of FIG. 3 throughin-situ synthesis on the inner surface 254 of the porous support 256.The method 280 generally includes preparing 282 the porous support 256to receive the membrane 252 layer. As described above, useful supports256 include porous alumina and stainless steel tubes or containers. Whenusing alumina supports 256, the ends 174, 176 of the alumina tubes areglazed to provide end supports 178, 180 and sealing surfaces 190, 192(see FIG. 2). Likewise, when using stainless steel supports 256,non-porous stainless steel tubes are welded onto the ends 174, 176 ofthe porous stainless steel tubes to provide end supports 178, 180 andsealing surfaces 190, 192. In either case, prior to use, the support 256is cleaned by brushing the inner surface 254 of the support 256,followed by immersing the support 256 in an ultrasonic bath of deionizedwater. The supports 256 are then boiled in distilled water and driedunder vacuum with heating (at about 373 K for about 30 min).

As indicated in FIG. 4, the method 280 also includes contacting 284 aninner surface 254 of the support 256 with zeolite precursors. Thezeolite precursors are provided as a synthesis gel comprised of silica,water, a source of metal ions (i.e., Al³⁺, B³⁺, Ge⁴⁺, Ga³⁺, Fe³⁺, etc.),and optionally, an organic template, such as tetrapropyl ammoniumhydroxide (TPAOH), tetrapropyl ammonium bromide (TPABr), tetrabutylammonium hydroxide (TBAOH), tetrabutyl ammonium bromide (TBABr),tetraethyl ammonium hydroxide (TEAOH) or tetraethyl ammonium bromide(TEABr) or combinations thereof. Referring to FIG. 2 and to FIG. 3, thesynthesis gel is placed in the interior portion 216 of the tubularmembrane assembly 170. The end supports 178, 180 are plugged with aninert material (e.g., PTFE) to form a container, and the gel is allowedto permeate the porous support 256. Ordinarily, the gel will thoroughlypermeate the porous support 256 in less than 24 hours when held at atemperature up to about 318 K. When possible, template-free synthesis isused because it costs less, does not use toxic amines, and does notrequire calcining, which may introduce cracks or other structuraldefects in the zeolite membrane 252 layer.

As shown in FIG. 4, the method 280 also includes crystallizing 286 thezeolite constituents to form a zeolite layer on the support 256 and,optionally, calcining 288 the resulting zeolite layer to remove anyorganic residues, including the organic template. The tubular membraneassembly 170 is placed in an autoclave and heated at a temperaturesufficient to induce zeolite formation, which is generally between about403 K and about 469 K. During heating, water within the synthesis gel isforced out of the interior portion 216 of the membrane assembly 170through the pores of the support 256, thereby forming a continuouszeolite layer on the inner surface 254 and within the pores of thesupport 256. The organic template molecules provided in the synthesisgel are trapped within the zeolite pore system and may also block largercavities in the zeolite membrane 252 layer. Thus, prior to calcining288, a zeolite membrane 252 layer without defects should be impermeableto gases, such as nitrogen, so that, as described below, vaporpermeation measurements can be used to evaluate zeolite membrane 252quality. Following crystallization 286, the uncalcined zeolite membrane252 layer is washed with deionized water and dried at 383 K for at least12 hours.

The contacting 284 and the crystallizing 286 steps (hydrothermalsynthesis) can be repeated one or more times to ensure that the zeolitemembrane 252 layer, after drying and before calcination, has therequisite quality. Following crystallization 286, the zeolite membrane252 layer is calcined 288 to remove the organic template and any otherresidual organic material. The organic template must be removed from thezeolite pores to obtain open, micro-porous membranes. Calcining 288generally comprises heating the zeolite membrane 252 layer at aprescribed rate until it reaches a desired temperature, e.g., about 750K or higher. This temperature is maintained for a sufficient amount oftime, e.g., about eight hours or more, to thermally decompose anyorganic material. Following thermal decomposition, the zeolite membrane252 is cooled at a prescribed rate to minimize thermal stresses in thezeolite layer. Ideally, the temperature profile is carefully controlledto ensure uniform heating and cooling within the zeolite membrane 252layer. Although uniform heating and cooling is generally best achievedusing relatively low temperature ramping (˜1 K/min), the method 280 mayemploy higher heating rates as long as care is taken to minimize localoverheating. Local overheating may result in partial degradation of thezeolite crystal structure and/or steam generation, which can causesiloxane bond hydrolysis and/or loss of aluminum from the zeoliteframework.

The zeolite membranes 252 of FIG. 3 can be characterized using manydifferent techniques. For example, the membranes 252 can becharacterized by X-ray diffraction (XRD) analysis of zeolite powderresidue sampled from the interior 216 of the membrane assembly 170 (FIG.2) following hydrothermal synthesis. This technique avoids destroyingthe membranes 252, and assumes that the zeolite membrane 252 layer andzeolite powder samples have the same crystal structure. A usefulapparatus for performing XRD measurements includes a Scintag PAD-Vdiffractometer, which uses a diffracted beam monochromator and aline-source X-ray beam of Cu Kα radiation from a standard 2 kW sealedtube. The X-rays are counted using a standard scintillation detector.Individual samples are ground to a fine powder and dispersed on a glassslide or packed into a cavity mount. The scan range (2θ) is typicallybetween about 2° and 50°, and phases are identified by comparingscattered intensity peaks with a library of known inorganic compounds. Auseful library of approximately 20,000 inorganic compounds is availablein a computer-readable format from the International Center ofDiffraction Data. Peak intensities and angles may also be calculatedfrom crystal structure data, if known.

The zeolite membranes 252 can also by characterized by pervaporatingcompounds of known sizes through the membrane 252 layer using the system100 shown in FIG. 1. Useful compounds include 2,2-dimethylbutane (DMB),which has a kinetic diameter (0.62 nm) that is larger than the XRD porediameter of the MFI structure. Other useful compounds include o-xylene,p-xylene, benzene, tri-isopropyl benzene (TIPB), which have kineticdiameters of 0.685 nm, 0.585 nm, 0.585 nm, and 0.85 nm, respectively.Xylene isomers are challenging to separate because they have similarphysical properties.

FIG. 5 shows a schematic view of an apparatus 310 that can be used tocharacterize zeolite membranes 252 by measuring single-gas permeationrates at various temperatures. As described below, with simplemodification the apparatus 310 can also be used to measure multi-gaspermeation rates. The vapor permeation apparatus 310 is similar to thepervaporation system 100 shown in FIG. 1, and includes a membrane module104′ having an inlet 106′, a first outlet 108′, and a second 110outlet′. A pressurized source 312 of gas 314 (H₂, He, CO₂, N₂, n-C₄H₁₀,i-C₄H₁₀, etc.) enters the membrane module 104′ inlet 106′ through afirst conduit 116′. A pressure regulator 316 and valve 318 located alongthe first conduit 116′, downstream of the source 312, provide coarsecontrol of the gas 314 flow rate. The first conduit 116′ is made of amaterial having good thermal conductivity (e.g., stainless steel), and asection 320 of the first conduit 116′ upstream of the membrane module104′ inlet 106′ is coiled to increase heat transfer area. Atemperature-controlled oven 322 encloses the membrane module 104′ andthe coiled section 320 of the first conduit 116′, and providessubstantially isothermal conditions within the module 104′ duringpermeation measurements.

As described above, the membrane module 104′ contains a tubular membraneassembly 170 (FIG. 2), which includes a zeolite membrane 252 layer and aporous support 256 layer (FIG. 3) that define a side wall portion of theassembly 170. Generally, a portion of the gas 314 entering the membranemodule 104′ passes radially through the zeolite membrane 252 and theporous support 256, and exits the module 104′ through the first outlet108′. The remainder of the gas 314 passes axially through the interior216 of the membrane assembly 170, and exits the module 104′ via thesecond outlet 110′. Second 128′ and third 324 conduits channel theresulting permeate 120′ and retentate 122′ streams, respectively, awayfrom the membrane module 104′. The second 128′ and third 324 conduitsconverge at a two-way purge valve 326, which allows venting of thepermeate 120′ stream or the retentate 122′ stream through a commonexhaust line 328 and a bubble flow meter 330.

As shown in FIG. 5, the apparatus 310 may include temperature andpressure sensors. The embodiment shown in FIG. 5 includes first 332 andsecond 334 thermocouples that are located in the second outlet 110′ ofthe membrane module 104′ and in the common exhaust line 328,respectively. A controller (not shown), which communicates with thefirst thermocouple 332, compares the temperature in the membrane module104′ with a desired set point, and adjusts the temperature of the oven322 in response to any offset. The embodiment also includes first 336and second 338 pressure sensors that are located, respectively, in apressure line 340 that communicates with the first conduit 116′ and thesecond conduit 128′, and in the first conduit 116′ immediately downstream of the pressure regulator 316 and valve 318. The first pressuresensor 336 is a differential pressure gauge that senses pressuredifferences between the gas entering the membrane module 104′ and thepressure of the gas in the permeate 120′ stream. To maintain a desiredpressure drop across the membrane module 104, the apparatus 310 includesa pressure regulator 342 (i.e., variable flow area valve) thatcommunicates with the first pressure sensor 336, and adjusts the flowrate of the gas permeate through second conduit 128′.

During a permeation measurement, gas 314 flow through second outlet 110′is stopped at the two-way purge valve 326, so that all of the gas 314entering the membrane module 104′ passes through the zeolite membrane252 layer and porous substrate 256 of the membrane assembly 170. Theresulting gas 314 flow rate through the zeolite membrane 252 layer ismeasured using the bubble flow meter 330. In most of the gas permeationexperiments, the pressure regulator 342 maintains a 138 kPa pressuredrop across the membrane module 104′. Single gas permeation rates areusually measured at two or more temperatures, e.g., at 300 K and 473 K.

With simple modification, the vapor permeation apparatus 310 shown inFIG. 5 can also be used to measure multi-gas permeation rates. Forexample, the apparatus 310 may include one or more metering pumps (e.g.,syringe pumps) that communicate with the first conduit 116′. During ameasurement, the metering pump injects a liquid-phase mixture into apre-heated carrier gas (e.g., He) flowing within the first conduit 116′.The liquid mixture vaporizes in the hot carrier gas, which transportsthe gas mixture into the membrane module 104′ inlet 106′. A portion ofthe gas mixture entering the membrane module 104′ passes radiallythrough the zeolite membrane 252 (FIG. 3) and the porous support 256,and exits the module 104′ through the first outlet 108′. The remainderof the gas 314 passes axially through the interior 216 of the membraneassembly 170, and exits the module 104′ via the second outlet 110′.Second 128′ and third 324 conduits channel the permeate 120′ andretentate 122′ streams, respectively, away from the membrane module104′.

In contrast to single-gas permeation measurements, the second 128′ andthird 324 conduits do not converge at the two-way purge valve 326 ofFIG. 5, but instead vent through separate exhaust lines and bubble flowmeters. As described above, the pressure regulator 342 can be used toimpose a desired pressure drop across the membrane module 104′, whichdrives diffusion through the selectively permeable portion 172 of themembrane assembly 170 (FIG. 2). Alternatively or additionally, theapparatus 310 may employ a sweep gas (e.g., He, Ar, etc.) to generate aconcentration gradient across the selectively permeable portion 172 ofthe membrane assembly 170. The sweep gas enters a cavity 218 formed byan inner surface 220 of the shell 182 and an outer surface 222 of themembrane assembly 170 through a port 360 in a body portion 184 of theshell 182. Note that the port 360 shown in FIG. 2 is sealed with aremovable plug 362.

The disclosed isomorphously substituted zeolite membranes will find usein many different processes. For example, the membranes can be used toseparate non-condensable gases. The thermal stability of the disclosedzeolite membranes makes them ideal for separating non-condensable gases,which are often available at high temperature. For example, theisomorphously substituted zeolite membranes of the invention could beused to separate H₂ from CO₂ in the water-gas shift reaction.

The membranes can also be used to separate condensable organic vapormixtures. The separation of condensable organic vapors often involvesseparating isomers that have similar relative vapor pressures.Typically, these separations are carried out using multiple distillationcolumns, require hundreds of stages, and are energy intensive.Isomorphously substituted zeolite membranes provide a much simpler, andless energy intensive separation process. For instance, as describedbelow in Example 4 and Example 7, the membranes of the invention havebeen used to separate n-C₄H₁₀/i-C₄H₁₀ mixtures and mixtures of xyleneisomers, respectively.

The removal of organic compounds or water from aqueous solutions isimportant for recovering valuable organic products from process streams,for recycling process water, and for treating wastewater. The disclosedhydrophobic or organophilic membranes can be used to separate suchorganic/water mixtures by pervaporation using the apparatus shown inFIG. 1. The hydrophobic, isomorphously substituted Ge-ZSM-5 membranespossess a different pore structure than silicalite-1 membranespreviously used for separating organic/water mixtures, and thus havedifferent permeation and adsorption properties. These membranes may beable to separate organic compounds from water with greater selectivitythan previously studied silicalite-1 membranes.

Because of their acid resistance, the disclosed isomorphouslysubstituted zeolite membranes may also be used to separate mineral acidsfrom water by pervaporation. Furthermore, the ability to vary theBrönsted acid strength of the disclosed membranes should prove useful inacid separations since water adsorption will likely vary with the numberof acidic sites within the framework structure and on the membranesurface.

The disclosed isomorphously substituted zeolite membranes can also beused in catalytic membrane reactors. Zeolite membranes have manyproperties that make them particularly useful as catalysts. First, it ispossible to introduce a large variety of cations, including protons,having different catalytic properties into the zeolite pore system.Second, zeolites exhibit a molecular sieving effect because of theirability to selectively adsorb molecules whose dimensions are below acertain critical size into their pore system. In shape-selectivecatalysis, the molecular pore structure and the presence ofcatalytically active sites is exploited to control reactionselectivity—i.e., to accelerate one of many potential reaction pathways.For example, ZSM-5 zeolites are used as shape selective catalysts in theconversion of methanol to gasoline and in the conversion of benzene andethylene to ethyl benzene. Generally, the catalytic activity andselectivity of zeolites depend on Brönsted acid strength. Since it ispossible to prepare membranes with different Brönsted acid strengths andwith different numbers of acidic sites, it may be possible to tailor thecatalytic activity and the selectivity of the disclosed membranes.Finally, the disclosed method of preparing zeolite membranes allows fordirect synthesis of the acid hydrogen form of the zeolite, which is muchsimpler than known synthesis techniques.

EXAMPLES

The following examples are intended as illustrative and non-limiting andrepresent specific embodiments of the present invention.

Example 1 Isomorphously Substituted Zeolite Synthesis

Zeolite membranes are prepared by in situ crystallization from zeoliteforming gels (zeolite precursors) on three types of porous supporttubes. The support tubes (OD=1.0 cm) comprise α-alumina with an innerlayer of γ-alumina having 5-nm diameter pores (0.70 cm ID, U.S. Filter),α-alumina with an inner layer of α-alumina having 200-nm diameter pores(0.70 cm ID, U.S. Filter), or porous stainless steel with an inner layerof stainless steel having 500-nm diameter pores (0.65 cm ID, MottMetallurgical Co.).

Alkali-free zeolite forming gels are prepared using silica sol (LudoxAS40) as the silicon source. Other silicon sources such astetraethyl-orthosilicate or fumed silica (Aeorsil-200) can also be used.A quaternary organic ammonium template, tetrapropyl ammonium hydroxide(TPAOH) is used to help stabilize and direct zeolite formation. Otherquaternary ammonium compounds such as tetrapropyl ammonium bromide(TPABr), tetrabutyl ammonium hydroxide (TBAOH), tetrabutyl ammoniumbromide (TBABr), tetraethyl ammonium hydroxide (TEAOH), tetraethylammonium bromide (TEABr) could also be used as templates. Forisomorphously substituted zeolite gels, Al(i-C₃H₇O)₃, Ge(C₂H₅O)₄,Fe(NO₃)₃, and/or B(OH)₃ are added synthesis solution, which is thenstirred for at least five minutes. Other solutions containing ionicAl⁺³, Ge⁺⁴, Fe⁺³, Ga⁺³, or B⁺³ could also be used. In some cases, thezeolite forming gel also contains sodium hydroxide.

The isomorphously substituted zeolite membranes containing B, Fe, Ge,Ga, and Al are prepared by in-situ crystallization on porous supports.Tubular supports 2.8 cm in length are used because they are commerciallyavailable and because they are well adapted for growing continuousfilms. One end of each tube is plugged with a polytetrafluoroethylene(PTFE) cap to form a container. A zeolite forming gel comprising silica,water, TPAOH, and a source of boron, aluminum, germanium, and/or iron isplaced inside the porous support container. The other end of the tube isplugged, and the container is left for periods up to 24 hours at roomtemperature. During this time, the porous support soaks up almost all ofthe synthesis gel. The container is again filled with gel, plugged, andplaced in an autoclave to allow the gel to crystallize. The firstcrystallization is carried out hydrothermally at 458° K for 24 hours.When the capped container is heated, water within the gel is forced topermeate through the pores of the support container, thereby forming acontinuous zeolite layer on the inner wall of the support container.

A synthesis using the same procedure but conducted at 453 K for 48 hoursis repeated until an uncalcined membrane, after drying at 373 K, isimpermeable to N₂ for a 138 kPa pressure drop at room temperature. Afterzeolite synthesis is complete, the membranes are washed, dried andcalcined to remove the organic template molecules from the zeolitepores. A computer-controlled muffle furnace with heating and coolingrates of 0.6 and 1.1 K/minute, respectively, is used for calcining themembranes. The maximum calcination temperature is 753° K, and themembrane is held there for eight hours and then stored at roomtemperature under vacuum.

A series of isomorphously substituted zeolite membranes having Si/Meratios of 100 was prepared using zeolite forming gels and synthesisconditions listed in Table 1. Each of the membranes was prepared onporous stainless steel supports. Because Fe³⁺ can be difficult toincorporate into the zeolite framework because of its large diameter,some extra-framework Fe³⁺ could be present within the membrane. However,the Fe-ZSM-5 membrane was prepared from a brown solution, but aftersynthesis, the membrane was white, and it remained white aftercalcination. This indicates that Fe³⁺ cations were likely incorporatedinto the framework positions; membranes containing extra-framework Fe³⁺are expected to be brown.

TABLE 1 Molar Compositions of Zeolite Precursors and SynthesisConditions for Preparing Isomorphously Substituted Zeolite Membraneswith Si/Me = 100 Crystallization Time (h) @ Membrane TPAOH Metal SiO₂H₂O Temp. (K.) # Layers silicalite-1 1.0 0    19.5 438 48 @ 458 2Al-ZSM-5 1.5 0.195 19.5 438 48 @ 458 2 Fe-ZSM-5 1.5 0.195 19.5 438 48 @458 2 B-ZSM-5 1.5 0.195 19.5 438 48 @ 458 2 Ge-ZSM-5 1.0 0.195 19.5 43824 @ 458 4

A second series of isomorphously substituted membranes having Si/Meratios ranging between 12 and 600 was prepared from the synthesis gelslisted in Table 2. The preparation conditions for these membranes aredescribed in Table 3.

TABLE 2 Molar Compositions of Synthesis gels for B-ZSM-5 and Al-ZSM-5Zeolite Membranes Membrane TPAOH SiO₂ B(OH)₃ H₂O NaOH Al₂O₃ Si/Me M1 2.0 19.46 0.39  438 2.0 0.0   50 M2  2.0 19.46 0.778 500 2.5 0.0   25M3  2.0 19.46 1.62  500 3.0 0.0   12 M4, M4a, 1.55 19.46 0.195 438 0.00.0   100  M4b, M7 M5, M8 2.22 19.46 0.778 500 0.0 0.0   25 M6, M9 4.4419.46 1.55  500 0.0 0.0     12.5 M10  1.0 19.46 0.0  438 0.0 0.0   ∞M11, M12  1.0 19.46 0.0  438 0.0 0.0162 600 

TABLE 3 Membrane Preparation Conditions for Molar Compositions shown inTable 2. Crystallization Time (h) @ Membrane Support Temperature (K.) #Layers M1 stainless 48 @ 458 4 M2 stainless 24 @ 458 4 M3 stainless 24 @458 5 M4, M4a, M4b stainless 48 @ 458 2 M5 stainless 24 @ 458 4 M6stainless 24 @ 458 5 M7 α-alumina 48 @ 458 2 M8 α-alumina 24 @ 458 4 M9α-alumina 25 @ 458 5 M10  stainless 48 @ 458 2 M11  stainless 48 @ 458 2M12  α-alumina 48 @ 458 2

Example 2 ZSM-5 Zeolite Structural Confirmation

MFI-type zeolites, such as silicalite-1 (pure silica) and ZSM-5(containing an isomorphously substituted element) have the samestructure with XRD pore dimensions of 0.53 nm×0.56 nm. To confirm theMFI-structure of the membranes prepared in Example 1, XRD powderpatterns were obtained for crystalline powders that were formed at thesame time as the membranes. This procedure avoids destroying themembranes; the membranes and powders were assumed to have the samecrystal structure. For all powders, the positions and the intensities ofthe diffraction peaks were identical to those reported for theMFI-structure. No additional peaks were observed, indicating that thepowders had the pure MFI structure.

A Scintag PAD V automated powder diffraction unit using a diffractedbeam monochromator and a line-source X-ray beam of Cu K-series radiationfrom a standard 2 kW sealed tube was used to characterize thesecrystals. The X-rays were counted using a standard scintillationdetector. Each sample was ground to a fine powder and dispersed on aglass slide or packed into a cavity mount. For phase identification, thescan range was typically 2° to 50° 2θ. Phase identification was based oncomparison of scattered intensity peaks with a standard file ofapproximately 20,000 known inorganic compounds. The standard file wasprovided by International Center of Diffraction Data (ICDD) (12 CampusBlvd, Newtown Square, Pa. 19073). Peak intensities and angles may alsobe calculated from crystal structure data, if known.

Additionally, the structure of membrane M4 of Table 2 was broken andcharacterized by XRD using the Scintag PAD V automated powderdiffraction unit. The sample tube was cut lengthwise and placed in aspecially made sample holder so that the membrane was in the correctcenter position of the diffraction instrument. The spot-source beam wascollimated so that only the portion of the tube in the correct positionwas exposed to the radiation. The positions and the intensities of allpeaks in the XRD pattern for the boron-containing zeolite membrane M4were also in agreement with those reported for MFI zeolite.

Membranes prepared in accordance with the invention on α-aluminasupports were also characterized by SEM. The SEM micrographs clearlyshow the presence of zeolite crystals on the alumina support. SEMphotographs were obtained with an ISI-SX-30 scanning electronmicroscope. Cylindrical membranes were broken and fragments selected assamples. Photographs were taken of the cross section and inner surfaceto show the structure and morphology of the membrane.

Example 3 Inductively Coupled Plasma Experiments

The boron content of the B-ZSM-5 membranes prepared in Example 1 wasverified by inductively coupled plasma after first dissolving thecrystals in hydrofluoric acid. The Si/B ratios in the zeolite powderswere determined to be similar to those in the zeolite forming gels. Forexample, zeolite powders formed from a gel having a Si/B molar ratio of50 were found to have actual Si/B molar ratios of about 60.

Example 4 Single Gas and Mixture Permeation Experiments

Single-gas permeation rates of H₂, N₂, and CO₂ were measured over arange of temperatures for most of the B-ZSM-5 membranes of Example 1, aswell as the Fe-ZSM-5, Ge-ZSM-5, Al-ZSM-5 and silcalite-1 membranes ofExample 1. In addition, single-gas and mixture permeances of n-C₄H₁₀ andi-C₄H₁₀ were measured for all of the membranes shown in Table 1-Table 3over the same temperature range. The single-gas permeation rates weremeasured by sealing the membrane in a stainless steel module withsilicone o-rings in a dead end mode. The pressure drop across themembrane was 138 kPa, and the permeate side pressure was 83 kPa. Theratio of single-gas permeances is referred to as the ideal selectivity.

Mixture permeances were measured in a continuous-flow stainless module,using He as a sweep gas. A 50/50 mixture of n-C₄H₁₀ and i-C₄H₁₀, with atotal flow rate of 40 cm³/minute, flowed axially inside of the tube, andthe permeate diffused radially outward. Silicone o-rings were used toseal the membrane inside the module. The module as wrapped in heatingtape and insulation. A temperature controller maintained the desiredtemperature based on a thermocouple placed at the axial outlet of themembrane. The permeate stream and the retentate stream were analyzedusing a HP 5890 gas chromatograph with a TC detector and a packed column(1% Alltech AT-1000 on Graph-GC). Each permeance was calculated from anaverage of four samples taken from the permeate and retentate streams.The calculated concentrations from the four samples at a given set ofconditions typically varied less than 2%. The volumetric flow rates ofretentate stream and the permeate stream were measured at roomtemperature and atmospheric pressure using soap-film flow meters. InTable 4, the n-C₄H₁₀/i-C₄H₁₀ separation selectivities are the ratios ofpermeances, and the log-mean partial pressure was used for thiscalculation.

Results of the permeation experiments are shown in Table 4 and FIG.6-FIG. 12. Table 4 shows single gas permeances and n-C₄H₁₀ and i-C₄H₁₀selectivities for B-ZSM-5 membranes that were prepared under differentconditions and that have various Si/B molar ratios. FIG. 6 and FIG. 7demonstrate the influence of boron substitution and support compositionon separation performance. FIG. 6 shows n-C₄H₁₀/i-C₄H₁₀ separationselectivity as a function of temperature for three B-ZSM-5 membranes(M4-M6) on stainless steel supports with different Si/B molar ratios(100, 25, 12.5) and for a silicalite-1 membrane on a stainless steelsupport. Similarly, FIG. 7 shows n-C₄H₁₀/i-C₄H₁₀ separation selectivityas a function of temperature for three alkali free B-ZSM-5 membranes(M7-M9) on α-alumina supports with different Si/B molar ratios (100, 25,12.5).

FIG. 8 compares n-C₄H₁₀/i-C₄H₁₀ mixture permeation rate as a function oftemperature for two alkali free B-ZSM-5 membranes (M4, M7) prepared onstainless steel (M4) and α-alumina (M7) supports. Both membranes haveSi/B molar ratios of 100. To study membrane stability, n-C₄H₁₀/i-C₄H₁₀mixture permeation rates were measured twice for the M4 membrane. Brokenlines show the second set of permeation rate measurements, which weretaken about 48 hours after the first set of permeation ratemeasurements.

FIG. 9 and FIG. 10 demonstrate the influence of support composition andbatch to batch variability on separation performance. FIG. 9 showsn-C₄H₁₀/i-C₄H₁₀ mixture permeance as a function of temperature for twoalkali free B-ZSM-5 membranes on stainless steel (M6) and α-alumina (M9)supports. Each membrane has a Si/B molar ratio of 12.5. FIG. 10 showsn-C₄H₁₀/i-C₄H₁₀ mixture permeance as a function of temperature for threealkali free B-ZSM-5 membranes (M4 a, M4 b, M4 c) prepared underidentical conditions (Si/B molar ratio of 100) on stainless steelsupports.

FIG. 11 demonstrates stability of an alkali free B-ZSM-5 membrane duringan extended permeation experiment. FIG. 11 shows n-C₄H₁₀/i-C₄H₁₀ mixturepermeance and separation selectivity as functions of time for B-ZSM-5membrane M9, which was prepared on an α-alumina support and had a Si/Bmolar ratio of 12.5. All of the measurements were made at 473 K. Asshown in FIG. 11, the i-C₄H₁₀ permeance decreased approximately 2% in 48hours, whereas the n-C₄H₁₀ permeance decreased by about 10% during theinitial 24 hours of the experiment and was almost constant during thenext 24 hours. The separation selectivity decreased from 59 to 56 after42 hours. There was no evidence of butane decomposition.

FIG. 12 demonstrates the relative influence of boron and aluminumsubstitution in the MFI structural framework. FIG. 12 showsn-C₄H₁₀/i-C₄H₁₀ separation selectivity as a function of temperature foralkali free B-ZSM-5 (M4, M7) and Al-ZSM-5 (M11, M12) membranes havingvarious Si/Me molar ratios (100, 100, 600, 600). The boron and aluminumsubstituted membranes were prepared on stainless steel (M4, M11) andα-alumina (M7, M12). For comparison purposes, FIG. 12 also showsn-C₄H₁₀/i-C₄H₁₀ separation selectivity as a function of temperature foran alkali free silcalite-1 membrane prepared on stainless steel.

TABLE 4 Singe Gas Permeances and Selectivities at 473 K for Me-ZSM-5Membranes Permeance × 10⁹ n/i-C₄H₁₀ (mol/m²/s/Pa Selectivity MembraneSi/Me H₂ CO₂ N₂ n-C₄H₁₀ i-C₄H₁₀ Ideal Separation M1 50  48  40  30  306.0 5.0 6.0 M2 25 110  89  70  68 23 3.0 3.4 M3 12 180 100  81  80 233.5 3.9 M4 100  40  32  20  20 1.0 20 22 M5 25  60  51  24  25 0.78 3235 M6 12.5  57  53  31  37 0.57 65 27 M7 100  95  88  46  59 2.2 27 39M8 25 162 162  79  60 2.5 24 27 M9 12.5 250 250 112 162 2.7 60 61 M10 ∞ 77  77  48  60 11 5.4 5.0 Fe-ZSM-5 100 100  70  40  42 6 7 8 Ge-ZSM-5100 140 120 100 110 7.5 14.7 14

In addition to the observations noted above, it appeared that for allsubstituted ZSM-5 membranes, single gas permeances at 473 K showed adecreasing trend as the kinetic diameter of the molecule increased. Allsubstituted membranes separated n-C₄H₁₀/i-C₄H₁₀, n-C₄H₁₀/H₂, andH₂/i-C₄H₁₀ mixtures, and separation selectivity seemed to depend on theidentity of the substituted metal or metalloid. However, no trend withacidity or hydrophobicity was observed. For most separations studied,the substituted membranes exhibited higher separation selectivity than asilicalite-1 membrane. The B-ZSM-5 membranes appeared to exhibit thehighest separation selectivity; of these, membranes prepared fromalkali-free gels exhibited the highest separation selectivity. Thehighest ideal selectivity at 473 K and 527 K was 60 and 24,respectively. For most B-ZSM-5 alkali-free membranes, n/i-C₄H₁₀ idealselectivity and separation selectivity increased with boron content, andmembranes prepared on α-alumina supports appeared to exhibit higherpermeance and separation selectivity than comparable membranes preparedon stainless steel supports. It appears that n-C₄H₁₀/i-C₄H₁₀ separationis due to differences in diffusion rates and adsorption coverage.

Example 5 Separation of Binary Mixtures of Normal Butane and Hydrogenand Isobutane and Hydrogen

Three isomorphously substituted ZSM-5 membranes of Example 1 (M4,Fe-ZSM-5, Ge-ZSM-5) were used to separate binary mixtures of normalbutane and hydrogen, and isobutane and hydrogen. For comparisonpurposes, a ZSM-5 membrane (M4) and a silcalite-1 membrane (M10) werealso used to separate the n-C₄H₁₀/H₂ and i-C₄H₁₀/H₂ mixtures. Permeationrates at temperatures ranging from 300 K to 523 K were measured using asystem similar to the apparatus described in Example 4. The single-gaspermeation rates were measured by sealing the membrane in a stainlesssteel module with silicone o-rings in a dead end mode. The pressure dropacross the membrane was 138 kPa, and the permeate side pressure was 83kPa. The ratio of single-gas permeances is referred to as the idealselectivity.

To measure mixture permeance, each of the binary mixtures was formed byevaporating n-C₄H₁₀ or i-C₄H₁₀/H₂ into a helium stream flowing withinthe tubular membrane. The membrane was located in a stainless steelmodule that was heated by heating tapes. Each hydrocarbon mixturecontained about 50/50 v/v mixture of n-C₄H₁₀/H₂ or i-C₄H₁₀/H₂. During anexperiment, both sides of the membrane were maintained at atmosphericpressure, and an argon sweep gas provided a driving force across themembrane by removing the permeating components. The permeate stream wasanalyzed using a gas chromatograph equipped with a flame ionizationdetector, as described in Example 4, and a log-mean pressure drivingforce was used to calculate permeance. Permselectivity was calculated asthe ratio of the permeances.

FIGS. 13-14 show the results of the permeation experiments. FIG. 13shows n-C₄H₁₀ separation selectivity as a function temperature for thesilcalite-1 and substituted ZSM-5 zeolite membranes. Because ofpreferential adsorption of normal butane, H₂ mixture permeance wassignificantly lower than its single gas permeance. As shown in FIG. 13,separation selectivity was highest at about room temperature, where then-C₄H₁₀ coverage (adsorption) was highest, and strongly decreased withincreasing temperature. At higher temperatures, the n-C₄H₁₀ coveragedecreased and could not effectively inhibit H₂ transport, resulting indecreased n-C₄H₁₀ separation selectivity at higher temperatures. Thisconclusion is supported by permeation measurements, which indicate thatseparation selectivity increases with increasing concentration ofn-C₄H₁₀ in the feed stream.

As shown in FIG. 13, n-C₄H₁₀/H₂ separation selectivity depended stronglyon the substituted metal or metalloid, and at room temperature,increased in the following order:Fe-ZSM-5<silicalite-1<Ge-ZSM-5<Al-ZSM-5<B-ZSM-5. Based on permeancemeasurements, the higher selectivity was the result of lower H₂permeance rather than higher n-C₄H₁₀ permeance. Although largedifferences in selectivity were probably the result of differentadsorption strengths of n-C₄H₁₀ within different zeolites, the order ofseparation selectivity did not correlate with acid strength orhydrophobicity/hydrophilicity.

FIG. 14 shows H₂ separation selectivity for i-C₄H₁₀/H₂ mixtures as afunction temperature for the silcalite-1 and substituted ZSM-5 zeolitemembranes. In contrast to the n-C₄H₁₀ /H₂ mixtures, H₂ permeated fasterthan i-C₄H₁₀ for all membranes, even at low temperatures. At lowtemperatures, H₂ permeance in the mixture was lower than its single gaspermeance, but was two to four times higher than H₂ permeance in theH₂/n-C₄H₁₀ mixture, indicating that i-C₄H₁₀ blocked H₂ permeation, butless effectively than n-C₄H₁₀. Thus, H₂ permeated faster than i-C₄H₁₀ inthe mixture. Like the n-C₄H₁₀/H₂ mixtures, increasing i-C₄H₁₀concentration in the feed inhibited H₂ permeation and thereforedecreased H₂/i-C₄H₁₀ separation selectivity.

As can be seen in FIG. 14, the H₂/i-C₄H₁₀ separation selectivity at 523K increased in the following order: silicalite-1<Fe-ZSM-5<Ge-ZSM-5<Al-ZSM-5<B-ZSM-5. The B-ZSM-5 membrane had thehighest selectivity because it had the lowest i-C₄H₁₀ permeance.

Example 6 Separation of N-Hexane From Binary Mixtures Containing 2,2Dimethylbutane, Benzene or Cyclohexane

Two of the B-ZSM-5 membranes (M6, M9) of Example 1 were used to separaten-hexane from binary mixtures containing 2,2 dimethylbutane, benzene orcyclohexane. Vapor permeation rates were measured using a system similarto system described in Example 4 at temperatures ranging from 373 K to524 K. The hydrocarbon mixture was evaporated into a helium streamflowing within the tubular membrane. The membrane was located in astainless steel module that was heated by heating tapes. Each of thebinary hydrocarbon mixtures contained about 50 vol. % n-hexane, and thefeed stream contained about 10 vol. % hydrocarbon and 90 vol. % helium.During an experiment, both sides of the membrane were maintained atatmospheric pressure, and a helium sweep gas provided a driving forceacross the membrane by removing the permeating components. The permeatestream was analyzed using a gas chromatograph equipped with a flameionization detector, as described in Example 4, and a log-mean pressuredriving force was used to calculate permeance. Permselectivity wascalculated as the ratio of the permeances.

Results of the permeation experiments are shown in Table 5 and FIGS.15-16. Table 5 lists mixture permeance and separation selectivity forboth B-ZSM-5 membranes (M6, M9). In addition to the binary mixturesdisclosed in Table 5, the B-ZSM-5 membranes were used to separate a50/25/25 mixture of n-hexane, cyclohexane, benzene mixture. Theresulting separation selectivity ranged from 790-800 at 373 K, which issignificantly higher than the separation selectivity for the binarymixtures shown in Table 5.

TABLE 5 Mixture Permeance and Separation Selectivity of 50/50 v/v normalhexane/organic mixtures for B-ZSM-5 Zeolite Membranes. α-Alumina SupportStainless Steel Support Permeance Selectivity Permeance Selectivity n-n-C₆H₁₄/ n- n-C₆H₁₄/ Organic C₆H₁₄ Organic organic C₆H₁₄ Organic organic2,2-DMB 1.5  0.65 2280  1.5  0.78 1950  Cyclo- 2.4 3.3 720 1.3 2.2 570hexane Benzene 2.4 5.5 440 1.0 2.6 370 Benzene + 2.2 2.8 790 1.5 1.8 800Cyclo- hexane (50/50)

FIG. 15 shows n-hexane/2,2-DMB permeance and separation selectivity asfunctions of temperature for both B-ZSM-5 zeolite membranes. Separationselectivity was highest (greater than 2000) at 373 K and decreased withincreasing temperature; but even at 523 K, the B-ZMS-5 membranesupported on stainless steel (M6) separated the n-hexane/2,2-DMB mixturewith selectivity of 72. FIG. 16 shows n-hexane/2,2-DMB permeance andseparation selectivity as functions of temperature for silicalite-1(M10) and one of the B-ZSM-5 zeolite membranes (M6). The B-ZSM-5membrane exhibited higher n-hexane permeance and lower 2,2-DMB permeancethan the silicalite-1 membrane. The B-ZSM-5 membrane exhibited thehighest n-C₆H₁₄/2,2-DMB separation selectivity, which was higher thanthe separation selectivity of the silcalite-1 membrane.

Example 7 Separation of P-xylene and O-xylene Mixtures

Two B-ZSM-5 zeolite membranes (BZ1, BZ2) were used to separate binarymixtures of p-xylene and o-xylene. The membranes were prepared in amanner similar to membrane M4 of Example 1, except membranes BZ1 and BZ2had four synthesis layers instead of two. The single-gas permeationrates were measured by sealing the membrane in a stainless steel modulewith silicone o-rings in a dead end mode. The pressure drop across themembrane was 138 kPa, and the permeate side pressure was 83 kPa.

Permeation rates for the xylene mixtures at temperatures ranging from373 K to 525 K were measured using a system similar to the apparatusdescribed in Example 4. Para-xylene/o-xylene mixtures were evaporatedinto a helium stream flowing within the tubular membrane to measuremixture permeance. The membrane was located in a stainless steel modulethat was heated by heating tapes. In most of the separations, the binaryp-xylene/o-xylene mixtures contained about 50/50 v/v mixture of the twoisomers. The partial pressure of each of the isomers varied amongseparations (0.4 kPa, 0.9 kPa, 2.1 kPa, 2.5 kPa). During an experiment,both sides of the membrane were maintained at atmospheric pressure, anda helium sweep gas provided a driving force across the membrane byremoving the permeating components. Helium flow rates for both the feedand the sweep gas were set at about 40 cm³/minute at STP using mass flowcontrollers. The permeate stream was analyzed using a gas chromatographequipped with a flame ionization detector, as described in Example 4,and a log-mean pressure driving force was used to calculate permeance.Permselectivity was calculated as the ratio of the permeances.

Results of the permeation experiments are shown in FIG. 17-FIG. 21. FIG.17 and FIG. 18 show, as functions of temperature, p-xylene/o-xylenesteady state fluxes and separation selectivities, respectively, forB-ZSM-5 zeolite membrane BZ1. The partial pressure of each of theisomers in the feed was 2.1 kPa. FIG. 19 and FIG. 20 show, as functionsof temperature, fluxes of p-xylene and o-xylene, respectively, forB-ZSM-5 zeolite membrane BZ2 and various feed partial pressures.Finally, FIG. 21 shows the resulting separation selectivity forp-xylene/o-xylene mixtures as a function of temperature for B-ZSM-5zeolite membrane BZ2 membrane and various feed partial pressures.

The above description is intended to be illustrative and notrestrictive. Many embodiments and many applications besides the examplesprovided would be apparent to those of skill in the art upon reading theabove description. The scope of the invention should therefore bedetermined, not with reference to the above description, but shouldinstead be determined with reference to the appended claims, along withthe full scope of equivalents to which such claims are entitled. Thedisclosures of all articles and references, including patentapplications and publications, are incorporated by reference in theirentirety for all purposes.

What is claimed is:
 1. An article of manufacture comprising a poroussupport and a zeolite membrane layer disposed at least in part on thesurface of en the porous support, the zeolite membrane layer beingformed by in-situ crystallization and comprising an isomorphouslysubstituted zeolite having a framework composition represented by theformula: [(y ₁ T ₁ ·y ₂ T ₂ ·y ₃ T ₃ . . . )O_(2(y) ₂ _(+y) ₂ _(+y) ₃_(+ . . . ))]; wherein T₁ is tetrahedrally coordinated Si, T₂ is atetrahedrally coordinated element and is B, or Ge, or combinationsthereof, T₃ is tetrahedrally coordinated Al, y₁, y₂, and y₃, arestoichiometric coefficients, and T₁/T₂ is between about 12 and about600.
 2. The article of manufacture of claim 1, wherein the membranelayer extends into pores of the porous support.
 3. The article ofmanufacture of claim 1, wherein the support comprises a container havingan interior surface, and the membrane layer is disposed on the interiorsurface of the container.
 4. The article of manufacture of claim 1,wherein the support is made of stainless steel, α-alumina or β-alumina.5. The article of manufacture of 1, wherein y₃ is substantially equal tozero.
 6. An apparatus for separating one or more components from amulti-component mixture, the apparatus comprising at least one membraneunit, a device for introducing the multi-component mixture into the atleast one membrane unit, and a device for removing the one or morecomponents from the membrane unit, wherein the at least one membraneunit includes a porous support and a zeolite membrane layer disposed atleast in part on the surface of the porous support, the zeolite membranelayer being formed by in-situ crystallization and comprising anisomorphously substituted zeolite having a framework compositionrepresented by the formula: [(y ₁ T ₁ ·y ₂ T ₂ ·y ₃ T ₃ . . . )O_(2(y) ₂_(+y) ₂ _(+y) ₃ _(+ . . . ))]; wherein T₁ is tetrahedrally coordinatedSi, T₂ is a tetrahedrally coordinated element and is B, or Ge, orcombinations thereof, T₃ is tetrahedrally coordinated Al, y₁, y₂, andy₃, are stoichiometric coefficients and T₁/T₂ is between about 12 andabout
 600. 7. The apparatus of claim 6, further comprising a pluralityof membrane units.
 8. A method of making an isomorphously substitutedzeolite membrane, the method comprising: preparing a porous support:contacting the porous support with an aqueous zeolite-forming gel, thegel being substantially free of alkali hydroxides and comprising silica,a quaternary organic ammonium template, and a source of ions, whereinthe ions are Al⁺³, Ge⁺⁴, Fe⁺³, Ga⁺³ or B⁺³ or combinations thereof;heating the support and the gel to form a zeolite layer at least in parton the surface of the porous support; and calcining the zeolite layer toremove the template.
 9. A method of making the article of manufacture ofclaim 1, the method comprising preparing a porous support: contactingthe porous support with an aqueous zeolite-forming gel, the gel beingsubstantially free of alkali hydroxides and comprising silica, aquaternary organic ammonium template, and a source of ions, wherein theions are Al⁺³, Ge⁺⁴, or B⁺³ or combinations thereof; heating the supportand the gel to form a zeolite layer on the porous support; and calciningthe zeolite layer to remove the template.
 10. The method of claim 9,further comprising washing and drying the zeolite layer.
 11. The methodof claim 9, further comprising repeating the providing and contactingsteps until the zeolite layer is substantially impermeable to N₂. 12.The method of claim 9, wherein the porous support is a container havingat least one opening and an inner surface.
 13. The method of claim 12,further comprising placing the gel in the container and capping the atleast one opening of the container.
 14. The method of claim 9, whereincalcining includes maintaining the porous support and the gel at atemperature between about 403 K and about 469 K for at least about twohours.
 15. The method of claim 9, wherein an acid hydrogen form of theisomorphously substituted zeolite membrane is synthesized directly fromthe gel.
 16. The method of claim 9, wherein the quaternary organicammonium template is tetrapropyl ammonium hydroxide, tetrapropylammonium bromide, tetrabutyl ammonium hydroxide, tetrabutyl ammoniumbromide, tetraethyl ammonium hydroxide or tetraethyl ammonium bromide orcombinations thereof.
 17. The method of claim 9 wherein the source ofions comprises at least one of Al(i-C₃H₇O)₃, Ge(C₂H₅O)₄, Fe(NO₃)₃, andB(OH)₃.
 18. A pervaporation method comprising the steps of: providing amembrane of claim 1; applying a multicomponent liquid mixture to oneside of the membrane; and applying a vacuum to the other side of themembrane, thereby causing permeation of at least one of the componentsof the mixture through the membrane.
 19. A vapor permeation methodcomprising the steps of: providing a membrane of claim 1; applying amulticomponent mixture to one side of the membrane, the mixture havingcomponents independently selected from the group consisting of vaporsand gases; and providing a driving force for permeation, thereby causingpermeation of at least one of the components of the mixture through themembrane.
 20. An article of manufacture comprising a porous support anda zeolite membrane layer formed by in-situ crystallization at least inpart on the surface of the porous support, the zeolite membrane layercomprising an isomorphously substituted zeolite having a frameworkcomposition represented by the formula: [(y ₁ T ₁ ·y ₂ T ₂ ·y ₃ T ₃ . .. )O_(2(y) ₂ _(+y) ₂ _(+y) ₃ _(+ . . . ))]; wherein T₁ is tetrahedrallycoordinated Si, T₂ is a tetrahedrally coordinated element and is B, Ge,Ga, or Fe or combinations thereof, T₃ is tetrahedrally coordinated Al,and y₁, y₂, and y₃ are stoichiometric coefficients and the thickness ofthe zeolite membrane layer is greater than about 30 microns.